Waste products decompose in landfills, and after the free oxygen in the landfill is depleted, the waste product decomposition generates methane gas. It is desirable to recover this methane gas for environmental and safety reasons, and because subsequent to recovery the gas can be used as a source of energy.
Accordingly, systems have been developed to extract the methane. One such system is disclosed in U.S. Pat. No. 5,458,006, which discloses that its system and other such systems typically include a plurality of vertical pipes, referred to as "well casings", that are vertically advanced at various locations into the landfill. The well casings are perforated along their lower-most segment, so that gas from the landfill can enter the casings. A network of horizontal pipes on or near the surface of the landfill interconnects the well casings, with a source of vacuum being in fluid communication with the network of horizontal pipes to evacuate the network and, hence, to evacuate methane gas from the well casings.
It happens that as methane gas is evacuated from a landfill, oxygenated air seeps back in. To avoid adversely affecting the generation of methane, however, the rate of oxygen inflow to the landfill must be controlled. Stated differently, to ensure continued methane gas production, the rate of gas extraction from the landfill and, thus, the rate of oxygen inflow to the landfill must be established to remain below a predetermined flow rate.
Not surprisingly, the methane gas extraction systems mentioned above typically provide for measuring gas flow rate through the well casings. In response to the measured rate, valves in the systems can be manipulated as appropriate to establish a desired flow rate through the well casings.
Several methods exist for measuring gas flow through the well casings. These methods typically involve measuring gas flow through a metering pipe that is in fluid communication with the well casing. One method simply involves measuring pressure at two points of the metering pipe that are longitudinally separated from each other. As is well understood, pressure head is inevitably lost in a pipe between an upstream location and a downstream location, with the magnitude of the pressure head loss being related to the gas flow rate through the pipe. Consequently, the pressure differential between any two longitudinally-spaced points in a pipe can be measured and then correlated to a gas flow rate.
Other methods for measuring gas flow rates through metering pipes include disposing an obstruction such as a an orifice or a pitot tube in the pipe and then measuring the pressure differential across the orifice or at the taps of the pitot tube. The pressure differentials are then correlated to gas flow rates in accordance with widely understood principles. The use of orifices advantageously permits the use of relatively short metering pipes, vis-a-vis metering pipes which simply measure head loss.
With particular regard to orifices, the '006 patent mentioned above teaches a metering pipe having an upstream segment and a downstream segment, with the segments being joined by a pipe coupler and with only the uppermost end of the downstream segment protruding through a bushing above the well casing. The remainder of the metering pipe, including an orifice used to generate pressure signals for calculating flow rate, is located in the well casing. As contemplated by the '006 patent, the metering pipe segments are made of polyvinylchloride (PVC), and the coupler is a PVC coupler formed with an internal ridge against which the pipe segments are advanced. The orifice is formed in a separate disc-shaped orifice plate made of plastic or steel which is sandwiched between the ridge of the coupling and one of the pipe segments. Thus, flow through the pipe does not encounter only a flat disc-shaped planar surface of an orifice plate, but the orifice plate circumscribed by the ridge of the coupling which rises from the plane of the plate. The combined effect of the ridge and plate can cause flow turbulence and thus decreased measurement accuracy.
Not surprisingly, therefore, the '006 patent teaches that the pressure taps which are formed in the metering pipe segments upstream and downstream of the orifice must be longitudinally distanced from the orifice plate by distances that are multiples of the diameter of the metering pipe, to ensure accurate flow rate measurement. For this reason, pressure lines must extend through the bushing located along the well casing, requiring modification of the bushing and rather elaborate pressure line-bushing fittings to ensure that gas does not leak between the pressure lines and bushing. As a further undesirable result, it will readily be appreciated that such a structure inhibits easily raising or lowering the metering pipe as might be required, e.g., to compensate for well casing settling. Like the '006 patent, U.S. Pat. No. 4,562,744 to Hall et al. teaches that pressure sensors in an orifice meter must be distanced from the orifice plate such that "minimum swirl or turbulence exists". As recognized by the present invention, however, a metering pipe orifice plate in combination with an internally ridged PVC coupling need not create turbulence. Further, the present invention recognizes that upstream and downstream pressure taps in a metering pipe containing an orifice need not be distanced from the orifice, but may be formed adjacent the orifice, thereby simplifying construction and design of the flow metering device while still ensuring accurate flow measurement.
Accordingly, it is an object of the present invention to provide a flow metering device for a landfill well that accurately measures gas flow through the well. Another object of the present invention is to provide a flow metering device for a landfill well that includes an orifice plate which cooperates with a pipe coupling. Still another object of the present invention is to provide a flow metering device for a landfill well which is easy to use and cost-effective.